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Author Topic: Building Steel Ground Bonding?  (Read 2327 times)

Mike Sokol

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Building Steel Ground Bonding?
« on: November 21, 2017, 09:20:09 am »

If you are dealing with a steel frame bulding and a diligent inspector, you will almost never find a ground that is only connected at the panel.  J-boxes are generally attached to steel and code requires them to be bonded to any grounding conductor in them.  As a matter of technicality, usually most red iron in a building is part of the grounding electrode and as such it is an acceptable ground.  If there is current in the grounding wire/building that is causing a voltage gradient that is causing trouble, then by definition that is objectionable current and a code violation.

Theory, code and real world are often at odds, though!

The neutral of course, is another story.

I'm going to make this a separate thread to expand on the discussion of multiple grounds being bonded to steel at various places around a building. I know that code allows for and even encourages this, but I've had this practice cause ground loop hum in a number of sites I've worked on. The solution was a separate green/yellow technical ground wire along with iso-ground receptacles. I'm not sure I totally understand all the code and practical implications, so we certainly should discuss it here.
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Jean-Pierre Coetzee

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Re: Building Steel Ground Bonding?
« Reply #1 on: December 01, 2017, 05:13:37 pm »

Last I checked code here in South Africa states that any metallic object that can be touches and might become electrified (including through lightning) should be bonded to ground. All building steel would fall under this since most buildings have Zinc roofing here.

I honestly feel that should this be the case then it would setup a quasi-grid grounding system which should decrease ground loops instead of cause them since all building steel would be bonded together allowing a possible lower impedance source to ground.

As far as I know though anything that isn't double insulated also happens to need a ground pin on the plug with a valid ground to the breaker panel and not simply being grounded to the building steel alone so there is that.

I can understand though that if there was shoddy bonding that there would be several different ground references and we all know that ground has different potential from point to point even in actual earth and that could cause a ground loop to occur quite easily.

Is it possibly an assume problem, bond everything to building steel assuming that all the building steel was correctly bonded together with sufficiently large enough conductors and therefore would be an excellent low impedance ground source but in actual fact there was no bonding or bad bonding of the building steel and therefore different ground potentials?

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Thomas Lamb

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Re: Building Steel Ground Bonding?
« Reply #2 on: December 06, 2017, 12:16:29 pm »

I just built a house and it was code to tie my ground into the rebar that is in the concrete. A Ufer ground? I had never heard of such a thing.  I just did what the inspector wanted and went on with life. I really don't understand how this helps anything.
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Stephen Swaffer

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Re: Building Steel Ground Bonding?
« Reply #3 on: December 06, 2017, 12:35:18 pm »

There are plenty of articles on this-but the basic explanation is that concrete always has some moisture content and is a relatively good conductor.  The concrete foundation of a building is kept in solid contact with the earth by gravity-ground rods can become "loose" in dry weather.

Strictly by the NEC, a UFER ground is one option you can use.  AHJs may make it mandatory-as is the case here in Iowa.  (It's not a difficult or expensive requirement-as long as they call the electrician before they pour the basement walls!)
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Mike Sokol

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Re: Building Steel Ground Bonding?
« Reply #4 on: December 06, 2017, 03:05:39 pm »

IIFC the UFER ground was invented as a way to get a decent ground in the sandy soil of a desert. As noted above, concrete always retains some moisture, so spreading out the "ground rod" over the square footage of an entire foundation helps couple your EGC to the earth.
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Kevin Graf

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Re: Building Steel Ground Bonding?
« Reply #5 on: December 07, 2017, 09:41:27 am »

The UFER ground was invented by Herbert G. Ufer in 1942. Note that it requires concrete mixed for the purpose and rebar installed correctly.
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Mike Nicolai

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Re: Building Steel Ground Bonding?
« Reply #6 on: December 08, 2017, 02:51:13 pm »

I know that code allows for and even encourages this, but I've had this practice cause ground loop hum in a number of sites I've worked on. The solution was a separate green/yellow technical ground wire along with iso-ground receptacles. I'm not sure I totally understand all the code and practical implications, so we certainly should discuss it here.

Article 250 of the NEC requires conductive panels/boxes/raceways to be bonded to ground. But 250.86, Exception 2, exempts short runs of conduits/boxes/raceways where the primary purpose of the conduit is for protection from physical damage. Think sleeves through walls/floors/etc or stub ups from wall jacks up to above a ceiling space where the cable run transitions to free air.

Additionally, 250.96(B) Isolated Grounding Circuits allows enclosures to isolated from raceway systems. You still have to ground the enclosure or receptacle to your isolated ground, but you do not have to bond it to your raceways, nor do you have to bond your insulated, isolated ground to your raceway system. Code does not require you bond raceways to any/all grounding conductors contained within -- just that you do in fact ground the raceway system by one mean or another.

In my projects, I require racks/receptacles to be isolated, bonded to an isolated ground feed from the isolation transformer if there is one the project. Otherwise those isolated grounds are bussed together at a designated location before being bonded to building ground. Then raceways and boxes are bonded to building ground, and I don't care where that happens so long as the electricians do it. Typically you don't have to get silly about running extra non-isolated ground conductors everywhere because a network of EMT conduits will naturally bond most/all of the raceway system together on its own.

Have to be a little careful not to compromise your isolated ground though. Not a problem for most connectors but Neutrik's earlier etherCon connectors had removable jumpers so you could pass the shield through the connector for Shielded CAT6 without bonding to the panel. Now their new CAT6A connectors are going to bond to whatever they're attached to whether you like it or not. Options for mitigating this include lifting the connectors from the panel with heatshrink sleeves/rubber gaskets/nylon screws, or transitioning your raceway into the box with a dielectric fitting and pulling an isolated, insulated ground conductor to bond your panel and connectors appropriately without intermingling them with your conduit ground.
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Mike Nicolai, CTS-D
Sarasota, FL

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Re: Building Steel Ground Bonding?
« Reply #6 on: December 08, 2017, 02:51:13 pm »


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